1. Field of the Invention
This invention relates to a method for determining the optimum inhibitor concentration in an aqueous fluid having dispersed therein a water-swellable, argillaceous material. More particularly, this invention relates to a method for determining the optimum inhibitor concentration required in an aqueous fluid to effect maximum inhibition of clay swelling and dispersion of argillaceous materials including clays as well as argillaceous sediments encountered in drilling operations. In this method measurements which are a function of the swelling, e.g. hydration, are made on the drilling fluid and utilized to ascertain the optimum inhibitor concentration.
2. Prior Art
Frequently in oil and gas well operations argillaceous sediments such as shales are encountered that swell and disperse when contacted with drilling fluids that are of inadequate nature to prevent such clay swelling and dispersion. Previously, drilling programs have been designed to maintain a predetermined inhibitor (such as potassium chloride, sodium chloride, calcium chloride, selective flocculants and coagulants) concentration based on shale dispersion tests, mineralogical analysis and experience in the area. Drilling programs designed in this manner have not proven to be satisfactory since shale dispersion tests are empirical and mineralogical analysis do not yield data that is useful in determining optimum inhibitor concentration. Since clay content and reactivity of argillaceous sediments to water vary from one formation to another in the drilling operation, the inhibitor concentration required for maximum clay swelling and dispersion inhibition will vary from one formation to another. Since inadequate inhibitor concentration can cause clay hydration and dispersion resulting in wellbore sloughing, out-of-gage hole, stuck pipe and fill during trips, optimization of inhibitor concentration to more closely match the nature of the argillaceous sediment can achieve maximum economic and drilling performance when drilling through troublesome shale zones.
The use of filtration tests for determining the stability of colloidal systems in drilling fluids containing clay-swelling inhibitors and protective colloids has been disclosed by Diener et al in U.S. Pat. No. 3,296,126. Walter in U.S. Pat. No. 3,989,630 has disclosed the testing of aqueous low solids drilling fluids containing hydration inhibitors such as calcium sulfate, calcium chloride, etc., to determine the filtration rates.
Apparatus useful for filtering drilling fluids has been described in a number of patents including U.S. Pat. No. 3,172,286, Grubb et al, U.S. Pat. No. 3,286,510, Parker, and U.S. Pat. No. 3,289,467, Parker et al.
In view of the numerous and well-known problems as described above which are associated with drilling operations carried out in areas where formations containing substantial quantities of shale are found, it is apparent that there is a definite need in the art for a reliable, convenient method for determining the optimum hydration inhibitor concentration needed in aqueous fluids to secure maximum clay swelling and dispersion inhibition.